1. Field of the Invention
The present invention relates to a sheet reversing device for reversing a sheet by feeding the sheet received into the device in a reverse direction and an image forming apparatus employing this sheet reversing device.
2. Description of the Related Art
There has been conventionally known a sheet reversing device 90 as shown in FIG. 10B incorporated into an image forming apparatus 99 shown in FIG. 10A. This image forming apparatus 99 is capable of printing both sides of sheets. Sheets fed one by one from a sheet cassette 991 by a feed roller 992 is conveyed to a photoconductive drum 995 via a conveyance path 993 at a specified timing after having the leading end thereof stopped by a registration roller pair 994.
On the other hand, an electrostatic latent image formed on the photoconductive drum 995 based on input image information is developed into a toner image with toner supplied from an unillustrated developing device. This toner image is transferred to the sheet fed from the sheet cassette 991. The sheet having the toner image transferred thereto is conveyed to a fixing roller pair 996 composed of a heating roller and a pressure roller to be heated and pressed, whereby the toner image is fixed to the sheet.
Upon printing both sides of a sheet, the sheet having a toner image fixed to one side is conveyed to the sheet reversing device 90 via a first reversing path 998 after having its conveying direction changed by a branch guide 997. In the sheet reversing device 90, the sheet has its conveying direction reversed to be conveyed to a second reversing path 999. Thereafter, this sheet is conveyed to the registration roller pair 994 via the second reversing path 999, is given a process similar to the printing process applied to the one side is applied to the other side and is then discharged to a discharge tray 999a via the branch guide 997.
The sheet reversing device 90 used in such an image forming apparatus 99 includes a reversing tray 91 and a switchback roller pair 92 provided at a conveyance opening 911 of this reversing tray 91 as shown in FIG. 10B. A lower roller 921 of the switchback roller 92 is rotatably mounted in the device. On the other hand, an upper roller 922 of the switchback roller pair 92 is rotatably supported on a supporting member 94 mounted on a conveyance guide plate 93 pivotally about a shaft 931. This arrangement is employed to enable the circumferential surface of the upper roller 922 to be stably pressed into contact with the circumferential surface of the lower roller 921.
Further, an upper guide member 95 is so mounted as to face the reversing tray 91 above a widthwise center position of the reversing tray 91 at the side of the switchback roller pair 92. A cover 951 is so mounted as to cover the upper surface of this upper guide member 95. Further, an opening 952 is formed in a widthwise central part of the upper guide member 95. A sheet detecting switch 96 including a detection bar 961 is mounted on the underside of the cover 951, and the detection bar 961 projects downward through the opening 952. The sheet detecting switch 96 is for detecting a sheet jam on the reversing tray 91.
A coil spring 97 is disposed between the supporting member 94 and the cover 951 and constantly biases the supporting member 94 downward. Accordingly, the upper roller 922 is constantly biased in a direction to be held in contact with the lower roller 921. Thus, a larger pressing force can act as compared with the case of using only the weight of the upper roller 922, wherefore even thick sheets can be easily conveyed.
According to the sheet reversing device 90 constructed as above, a sheet having an image formed on one side is conveyed to the reversing tray 91 through the conveyance opening 911 by the switchback roller pair 92. At this time, the detection bar 961 of the sheet detecting switch 96 is touched and rotated by the sheet being conveyed, whereby the presence of the sheet is detected. When the trailing end of the sheet reaches a nip of the switchback roller pair 92, the switchback rollers 92 are rotated in reverse directions to convey the sheet to the conveyance path from the reversing tray 91. Thereafter, an image is formed on the other side of the sheet to complete a duplex printing process, and is discharged to the external discharge tray 999a. 
In the above sheet reversing device 90, upon duplex printing, a sheet having one side printed passes through the first reversing path 998 and is conveyed onto the reversing tray 91 via the switchback roller pair 92. Thereafter, the sheet is pulled out from the reversing tray 91 by rotating the switchback rollers 92 in reverse directions and conveyed to the second reversing path 999.
Here, the sheet may be curled, for example, due to heat in the fixing roller pair 996 at an upstream side and the leading end thereof may be warped upward. In this case, the leading end of the sheet may face an upper side of a bottom end edge 998a of the first reversing path 998 to be guided to the lower end edge 998a and move toward the first reversing path 998 when the sheet is fed in the reverse direction from the reversing tray 91. If the sheet moves toward the first reversing path 998, the sheet cannot be reversed, whereby not only duplex printing cannot be performed, but also the sheet may be jammed in the first reversing path 998.